Here is how I would approach it, in general:
1 Check compression. Easier said than done on cylinder #4 because of the lower cowl. They had a special, small, screw-in gauge that fit in there.
2 Drain and fill the gear case. Mercs built prior to 1976 had steel shafts that rusted, causing the oil seals to fail, water intrusion into the gear case and subsequent gear case failure.
3 check the spark. You should have a gigantic whack on all four.
4 Clean the carbs. Again, easier said than done on this engine, as the mag is in the way. Drop it out of the way using quarter-inch drive sockets, extensions and a flex joint.
5 Replace the water pump impeller, if you are still good at this point. Any of those Merc impellers are suspect if more than two years old. They might start out okay and then fail in a heart beat.
Good-running, strong engine, when it’s right.
If it has been in salt water, do not pass go; give it to a scrapper. Save the ignition parts, though. They are hard to come by.
Long live American manufacturing!
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